Contact printing using high volume presses is commonly employed to print a large number of copies of an image. A contact printing press typically utilizes a printing plate to apply a colorant to a surface to form an image thereon. The surface can form part of a receiver media (e.g. paper) or can form part of an intermediate component adapted to transfer the colorant from its surface to the receiver media (e.g. a blanket cylinder of a press). In either case, a colorant pattern is transferred to the receiver media to form an image on the receiver medium.
Printing plates typically undergo various processes to render them suitable for use in a printing press. For example, exposure processes are used to form images on an imageable surface of a printing plate that has been suitably treated so as to be sensitive to light or heat radiation. One type of exposure process employs masks. The masks are typically formed by exposing highly sensitive film media using a laser printer known as an “image-setter.” The film media can be additionally developed to form the mask. The mask is placed in contact with a sensitized printing plate, which is in turn exposed through the mask. Printing plates exposed in this manner are typically referred to as “conventional printing plates.” Some conventional lithographic printing plates are sensitive to radiation in the ultraviolet region of the light spectrum.
Another conventional method directly forms images on printing plates through the use of a specialized imaging apparatus typically referred to as a plate-setter. A plate-setter in combination with a controller that receives and conditions image data for use by the plate-setter is commonly known as a “computer-to-plate” or “CTP” system. CTP systems offer a substantial advantage over image-setters in that they eliminate film masks and any process variations associated therewith. Printing plates imaged by CTP systems are typically referred to as “digital” printing plates. Digital printing plates can include photopolymer coatings (i.e. visible light plates) or thermo-sensitive coatings (i.e. thermal plates).
In many printing processes, a plurality of printing plates is used to apply different colorants to a receiver media. Typically, each printing plate applies a different colorant to the receiver media. In this way, the printed image formed on the receiver media can contain different colors. Each of the printing plates must be registered with respect to one another to form a printed image having a desired visual quality. Regardless of the manner by which an image is formed on a printing plate, it needs to be accurately positioned on the printing plate to achieve a desired registration with the images formed on other associated printing plates.
In some cases, registration features are formed in a printing plate to help register the printing plate on a printing press. The registration features can be formed by various processes including processes adapted to form perforations in the printing plate. A set of perforations can be used to define registration features comprising locating holes or locating channels adapted for providing a desired alignment with a corresponding set of registration features on a printing press. Accurate registration requires that the registration features formed on a printing plate be registered with the images formed on the printing plate. In some cases, the image forming process and the registration feature forming process are conducted by different apparatus. In other cases, the image forming process and the registration feature forming process are conduced by the same apparatus. In some cases, the image forming process precedes the registration feature forming process while in other cases, the opposite occurs. In some cases, a registration feature formed on a printing plate is employed to assist in the accurate placement of an image on the printing plate. In other cases, an image formed on a printing plate is employed to assist in the accurate placement of a registration feature on the printing plate.
In many cases, one or more edges of a printing plate are used for registration purposes during a processing of the printing plate. For example, during some processes, a printing plate is aligned on a support surface of an apparatus by bringing one or more of the plate edges known as “registration edges” into contact with various registration members. Various groupings of registration members are often employed to register printing plates to the support surface. Once a required contact is established between the printing plate and the registration members, the printing plate is deemed to be in a required registration for a subsequent processing such as the forming of an image or a registration feature. Failure to establish the necessary contact between the printing plate and the registration members can introduce registration errors during the subsequent processing. The failure to establish the necessary contact between the printing plate and the registration members is referred to as “misregistration.” Registration errors can lead to reduced quality in the finished printing plate and adversely impact the productivity of the plate making process.
Various problems are associated with positioning a printing plate against a plurality of registration members. Today's automated printing plate processing systems require printing plates to be carefully registered against various registration members without deforming the printing plates and while maintaining high processing throughputs. Printing plate deformations can take various forms including various buckling modes which can cause the entrapment of “bubbles” or other similar distortions that can lead to printing plate surface deviations during a subsequent processing operation. Printing plate deformations can also include deformations in an edge of a printing plate which lead to printing plate positional errors during a subsequent processing operation. Edge deformations can arise for different reasons, including excessive contact stresses arising during the positioning of the printing plate against the registration members.
The likelihood of printing plate deformations can increase as the size of the printing plate increases. For example, as a printing plate increases in size, so do the frictional forces between it and a support surface onto which it is positioned during the registration process. This in turn, leads to the need for larger positioning forces to register the printing plate against the registration members, thereby increasing the likelihood of plate deformations or conversely, misregistrations when the printing plate is not properly positioned against one or more of the required registration members.
Various conventional printing plate registration detection systems have been employed to help detect whether or not a required registration of a printing plate has been achieved. For example, in commonly-assigned U.S. Pat. No. 6,510,793 (Kerr et al.), which is herein incorporated by reference, describes a electronic printing plate registration system in which registration is established when the edges of a printing plate contacts all of three electrically conductive members to create a short between all of these conductive members. In one embodiment, Kerr et al. teach the use of a signal generator that generates an electrical signal at each of two of the three conductive members which act as “emitter” members. An electrical short detection system employs a short detector that senses both the electrical signals at the remaining third conductive member which acts a “receiver” member. In this regard, the electrical detector is adapted to detect both the electrical signals provided by the two “emitter” conductive members. Kerr et al. teach the use of two signals having different characteristics (e.g. frequency) to determine whether a misregistration is created by an absence of contact between the printing plate and a particular one of the two emitter members. The plate detection system described by Kerr et al. is an example of a conventional detection system in which the detection of contact between the printing plate and any given one registration member is dependent on the presence contact between the printing plate and another of the registration members. There is a need for improved methods and apparatus for properly registering one or more printing plates during a printing plate processing operation.
There is a need for an imaging apparatus with improved printing plate registration abilities.
There is a need for a perforation apparatus with improved printing plate registration abilities.